An attempt has been made to establish experimentally the temperature along the metastable eutectic in the Fe-C-Si system, down to the invariant three phase eutectic involving austenite, cementite and an iron silico-carbide. This was carried out by standard thermal analysis using cups with a coating rich in tellurium that is known to impede graphite growth. It turned out that evaluating the metastable temperature is possible up to a silicon contents of 3.8 wt.%, beyond which the driving force for graphite growth becomes so high that the development of stable eutectic cells can no more be hindered. This is quite unfortunate for melt control on the foundry floor but triggers further study for understanding and describing graphite shape in cast irons.